Recently it has become of increasing interest to provide plastic films which are coated with metal oxides and like metal compounds as wrapping for foodstuffs and other materials which require limited access of moisture or oxygen to the packaged product or as to which it is desirable to limit the permeability of the film to fluids from the packaged product. Such coatings are also of interest as optical coatings for changing or establishing the appearance of a web of plastic.
In the past the coating systems for such purposes have been relatively slow, even if a roll-to-roll arrangement was proposed. Generally the time-consuming nature of the process depended upon the fact that the metal was required to react in the reactive atmosphere before it reached the web of plastic film. Alternatively, systems in which the metal was deposited upon the film and then reacted with the reactive atmosphere could not provide adequate time for the reaction to complete while ensuring sufficient speeds of the web.
Typical of the films which attempts were made to coat with metal compounds in the manner described were polyethylene, polyethyleneterephthalate and polyphenyleneoxide. The coatings were metal oxides and were intended to provide barriers to oxygen and water-vapor transport. The products were transparent films and were used for food packaging, particularly for foods sensitive to oxygen and water vapor.
The deposition of the coating onto the film generally involved plasma vapor deposition, for example, crucible evaporation, electron gun evaporation and sputtering or chemical vapor deposition (CVD).
The most common metal compounds were the oxides and nitrides and various metals could be deposited. (See the commonly-owned copending application Ser. No. 08/025,514 dated 2 Mar. 1993 and Ser. No. 08/222,690 filed 4 Apr. 1994.)
All of these earlier systems required considerable time to allow the metal to react with the gaseous environment even where the latter was activated by ionization or some other excitation to permit reaction with the metal.
In plastic film coating utilizing continuous plants with conventional configurations, it has been found to be impossible to allow the requisite reaction of the coating metal with the reactive atmosphere before formation of the coating since, for practical reasons, the web must travel at a speed of meters per second. The problem is exacerbated by the fact that satisfactory coatings require thicknesses of hundreds of angstroms of the dielectric, i.e. the metal oxide or nitride. In many cases the metal deposition rate is too high to obtain complete reaction, especially where the reaction is carried out at a pressure below 10.sup.-3 torr.